Such as a television receiver, image display apparatuses like a CRT (Cathode Ray Tube) display and a LCD (Liquid Crystal Display) have been widely spread. In recent years, demands on large pictures for home theater purpose monitors, large-screen televisions, and the like, have been increased.
As typical means for forming a large picture, a projection type image display apparatus has been widely known, which performs image formation with an illumination light from a projector on a screen to display an image. Also, a projection type image display apparatus may be roughly classified into two types of front projection type that projects an illumination light from a projector on a front side of a screen facing an observer, and rear projection type that provides projection on a transmission type screen and a formed image is observed from a rear side.
The basic structure of a rear projection type image display apparatus is that a small, high luminance image is enlarged and projected on a rear surface side of a transmission type screen viewed by an observer. An image source may employ a CRT (Cathode Ray Tube) projector, however, in recent years, a configuration using a spatial modulation element such as a liquid crystal projector is being developed. Also, images respectively from monochromatic projectors of three primary colors may be superposed on a rear surface of a transmission type screen to form a color image.
For a rear projection type image display apparatus, one of major design guidelines is that a front gain is increased and projection is provided on a screen in order to increase the luminance at the front side and to attract the observer's eye. Also, in order to prevent a “hot spot” phenomenon in which a light source becomes noticeable in a projection image on a screen, a transmission type screen is typically constituted by a diffusing plate. Distribution of light is performed by diffusing light, which has formed an image, in various directions, so as to form a good image and supply the image to an observer.
For example, a rear projection type screen is suggested, in which a lenticular lens with its horizontal direction being a longitudinal direction is formed at a light incident surface, a light non-transmitting portion in which a black stripe with its horizontal direction being a longitudinal direction is located near a focal point of the lenticular lens, and a lenticular lens with its perpendicular direction being a longitudinal direction is formed at a light outgoing surface, so as to realize a fine lenticular lens pitch not causing a moire disturbance and to reduce reflection of outside light (for example, refer to Patent Citation 1).
Also, a light diffusing sheet and a rear projection type screen are suggested, in which light transmitting layers made of polycarbonate or MS resin having a low hygroscopicity are provided on both sides of a diffusing layer essentially consisting of PMMA, whereby a warp due to moisture absorption hardly occurs and a good diffusion characteristic is exhibited (for example, refer to Patent Citation 2).
Also, a rear projection screen purpose diffusion structure plate is suggested, in which a plurality of reflecting mirrors having a parabolic cylinder shape are formed in parallel between an incident surface and an outgoing surface of a diffusing sheet, to make a contribution to sharpening of an image (for example, refer to Patent Citation 3).
Also, a transmission type screen that can be used for a rear transmission type image display apparatus is suggested, in which a plurality of microlenses that collect an incident light, a black matrix provided on the outgoing side of the microlenses and having an aperture part in an optical path of the transmitted light, a diffusing portion having a recess at a portion corresponding to the aperture part to diffuse the transmitted light of the microlenses, are provided, thereby providing a good viewing angle characteristic, and a good light utilization efficiency (for example, refer to Patent Citation 4).
Also, recently, a multi-projection system (refer to FIG. 16) is known as one of trials for increasing the projection region and resolution, in which projectors are arranged in an array or the like, so that a plurality of images are partially superposed on each other and are projected on a screen to display a large image.
For example, a liquid crystal display multi-projection apparatus is suggested, in which an optical axis of a projection image from each image projection unit is determined so as to be substantially parallel to the projection screen, and an image is reflected by a reflecting mirror near the projection screen to be displayed on the projection screen, thereby providing a thin configuration (for example, refer to Patent Citation 5).
Also, a multi-projection type display apparatus is suggested, in which the brightness of a part of an image signal, corresponding to each image, superposed on an adjacent image is electrically finely adjusted to project an image corresponding to a plurality of image signals, and the brightness of the superposed part superposed on the adjacent image is optically attenuated with a predetermined smooth distribution to smoothen the brightness at the superposed part, thereby making the superposed part of the image less noticeable though being adjacent (for example, refer to Patent Citation 6).
Also, a large screen display apparatus is suggested, in which an upper projector for projecting an image signal on an upper surface region of a screen at an acute angle is installed at an upper front portion of the screen, and a lower projector for projecting an image signal angle on a lower surface region of the screen at an acute is installed at a lower front portion of the screen, so that a person is prevented from being obstruct because the person getting into between the screen and the projector when the person views the image on the screen (for example, refer to Patent Citation 7).
In the front projection type, if a picture is enlarged, there is a problem that the shadow of an observer may be easily reflected on the screen. In light of this, if the region is enlarged, such as when projectors are arranged in an array to realize multi-projection, the inventors of the present invention consider that it is desirable to employ the rear projection type.
Meanwhile, a projection light from a projector, such as a LCD (Liquid Crystal Display) or a CRT, is basically divergent from a central portion to a peripheral portion, and partially extremely, high-directional light is incident on the transmission type screen. As described above, through the appropriate distribution of light using the diffusing plate, an observable image can be at least obtained on the screen. However, since the projection light is divergently incident and the peripheral portion has the outward directional property, when being observed from the front side of the screen, there is a problem that the peripheral luminance becomes darker than the central luminance. For example, when being observed obliquely, unevenness of brightness of the picture is generated such that a near edge portion becomes bright whereas a far edge portion becomes dark.
Even though a diffusing plate is provided in the path of the projection light, or the projection screen itself is constituted by a diffusing plate for diffusing the light beam, which has formed an image, in various directions to equalize the luminance, it is difficult to have a maximum gain at the front side of the projector and to avoid generating a phenomenon that a gain becomes small as an angle increases with respect to a light beam direction with the maximum gain obtained. As a typical index for the change in screen gain, an angle at which the maximum gain becomes half, that is, a “half-power angle” is used. As the half-power angle increases, a good large picture with a wide viewing angle can be obtained.
In the case of the front projection type image display apparatus, the half-power angle is 80 degrees, thereby obtaining diffusion in a wide range. In contrast, in the case of the rear projection type image display apparatus, because of the design guideline that the luminance at the front side is increased (described above), the half-power angle is merely about 45 degrees at maximum even when a highly diffusive screen is used. In other words, through the rear projection, even though a diffusing plate for simple diffusion is used, it is difficult to perform perfect diffusion, causing a high front gain.
Having a small half-power angle means that the amount of attenuation in luminance becomes radical when an image is viewed from a position other than the front side (that is, when an angle defined by an axis perpendicular to the screen and a line of sight is large), the color may be changed, the unevenness of color may be generated, or the unevenness of luminance may be seriously increased, depending on the location of the image displayed on the screen. Thus, when the multi-projection system in which the projectors are arranged in an array is constructed, if the rear projection type projector causing a half-power angle to be small is used, the front gain of each outgoing light on the screen becomes higher than the peripheral gain, thereby possibly becoming a factor of degrading the image quality.
Using a Fresnel lens as means for solving the problem of the half-power angle in the rear projection type image display apparatus, is known in the art.
For example, a projection screen is suggested, in which a Fresnel lens sheet, a first lenticular sheet having a convex cylindrical lens arranged in a first direction, and a second lenticular sheet having a convex cylindrical lens arranged in a second direction orthogonal to the first direction, are arranged in that order (for example, refer to Patent Citation 8), thereby extending the sight in two directions of left-right (horizontal) direction, as well as up-down (perpendicular) direction, and strictly controlling the outgoing range and contrast of display image light.
However, when the projection is provided on the screen through the Fresnel lens, although the brightness at a “blending region” (refer to FIG. 16), where projection planes of the projectors are superposed, can be equalized, since the Fresnel lens has a limit for a corresponding incident angle gentleman, the directional property of the outgoing light cannot be eliminated. Also, because of the limit for the corresponding incident angle, the installation position of the projector with respect to the Fresnel lens is restricted strictly. Further, if the Fresnel lens is used, it is difficult to form a large screen, and hence, there is a problem that the manufacturing cost may become high.
Also, as another problem of the rear projection type image display apparatus, since the illumination light from the projector is directed to an observer through the transmission type screen, there is a problem that a direct light is transmitted and observed.    Patent Citation 1: Japanese Unexamined Patent Application Publication No. 11-344769    Patent Citation 2: Japanese Unexamined Patent Application Publication No. 2002-207253    Patent Citation 3: Japanese Unexamined Patent Application Publication No. 2005-24828    Patent Citation 4: Japanese Unexamined Patent Application Publication No. 2005-128352    Patent Citation 5: Japanese Unexamined Patent Application Publication No. 9-265132    Patent Citation 6: Japanese Unexamined Patent Application Publication No. 2001-34304    Patent Citation 7: Japanese Unexamined Patent Application Publication No. 2004-247868    Patent Citation 8: Japanese Unexamined Patent Application Publication No. 2004-215715